1. Field of the Invention
The present invention relates to a displaying device and a lighting device employing an organic electroluminescence element (hereinafter, referred to as “organic EL element”) of high efficiency.
2. Background Art
An organic EL element is a light-emitting device in which electrons and positive holes are injected into an organic layer to emit light. Because of self-luminosity, it is characteristic of the organic EL element to give high contrast views and to have a very thin light-emitting part. Accordingly, it is being studied to use the organic EL elements in practical application to thin flat displaying devices and thin flat lighting devices. These devices are employable for cellular phones, thin TV sets and household lighting equipments.
In the organic EL element, a transparent electrode is provided as an electrode on the anode side. The transparent electrode is normally a layer made of indium tin oxide (hereinafter, referred to as “ITO”), which is indium oxide doped with tin as a dopant. This electrode must have not only enough electrical conductivity to function as an electrode but also transparency to light emitted from the light-emitting layer.
A displaying device employing the organic EL element (hereinafter, referred to as “organic EL display”) preferably has a top emission type element structure, in which a TFT (thin film transistor) array is provided on the substrate side opposite to the light-emitting side so as to enlarge the imaging area. In the top emission type element structure, an upper electrode placed on the light-emitting side needs to transmit light. For that reason, in a normal displaying device, the light-transmitting electrode is generally made of ITO. However, in producing the organic EL display of top emission type, the upper electrode on the light-emitting side must be formed after the organic light-emitting layer is formed. Accordingly, it is difficult to adopt an ITO electrode in the top emission type organic EL display because a sputtering process or a high-temperature treatment is necessary to form the ITO electrode.
Meanwhile, a lighting device employing the organic EL element (hereinafter, referred to as “organic EL lighting device”) is required to be thinner and to have a larger light-emitting area. However, if a commonly used ITO layer is used as the light-transmitting electrode, it is difficult for the EL lighting device to emit light evenly from the whole surface. The reason of that is because the ITO layer has 100 times or more as large resistivity as a normal metal electrode and hence the resistance in plane increases according as the light-emitting area increases.
Further, indium, which is a material of the ITO layer, is such a rare metal that exhaustion thereof in the future is now brought into a serious problem. Accordingly, there is a fear that it will be difficult to procure indium in the future.
To solve the above problems about the ITO layer, it is being studied to employ a thin metal layer as the light-transmitting electrode.
Generally in producing an organic EL display, a very thin semi-transparent metal electrode having a thickness of a few nanometers to dozens of nanometers is formed by vacuum deposition or the like, which less damages the organic light-emitting layer. However, the semi-transparent metal electrode considerably lowers light-transparency to impair brightness of the resultant EL display seriously. Accordingly, in order to obtain emission in high brightness, it is necessary to increase the device-driving voltage. However, if the voltage is increased, heat is unfavorably generated to significantly shorten the working life-time of the display. Further, it is difficult to evenly produce the very thin semi-transparent metal electrode if the display has a large displaying surface. This means that a process margin in the procedures of forming the upper electrode becomes so small that the yield in mass-production is lowered.
Also in producing an organic EL lighting device, it is difficult to evenly produce the very thin semi-transparent metal electrode if the device has a large light-emitting surface. In addition, if the electrode has a thickness enough to ensure high electrical conductivity, the light-transparency is impaired.
As described above, it has been difficult for the conventional light-transmitting electrode such as an ITO layer or a thin metal layer to be on a high level of both light-transparency and electrical conductivity.
For the purpose of solving the above dilemma, there is an attempt to reduce resistance of the ITO layer. For example, JP-A 2005-332705(KOKAI) discloses a technology for forming an ITO layer having small resistance. In the disclosed technology, a transparent substrate is first coated with a metal mesh electrode having a thickness of not more than 15 μm, a line width of not more than 25 μm and an opening diameter of 50 μm to 2.5 mm. The openings of the mesh are then filled with a transparent resin film, and then an ITO layer is provided on the whole top surface. However, even in this technology, the metal mesh electrode serves as only an assistant to the ITO layer and hence does not essentially solve the above problems.